Re: pg_migrator issues

Bruce Momjian wrote:
> pg_migrator has become more popular recently, so it seems time to look
> at some enhancements that would improve pg_migrator.  None of these are
> required, but rather changes that would be nice to have:
> 
> 1)  Right now pg_migrator preserves relfilenodes for TOAST files because
> this is required for proper migration.  Now that we have shown that
> strategically-placed global variables with a server-side function to set
> them is a viable solution, it would be nice to preserve all relfilenodes
> from the old server.  This would simplify pg_migrator by no long
> requiring place-holder relfilenodes or the renaming of TOAST files.  A
> simpler solution would just be to allow TOAST table creation to
> automatically remove placeholder files and create specified relfilenodes
> via global variables.

Getting rid of the need for placeholders is a good idea.  +1 on getting
TOAST tables created with the correct relfilenode from the start.  I
don't know that preserving any other relfilenode is useful; however if
it means you no longer have to rename the files underlying each table,
it would probably also be a good idea.  (I don't know how does
pg_migrator deal with such things currently -- does it keep a map of
table name to relfilenode?)

> 2)  Right now pg_migrator renames old tablespaces to .old, which fails
> if the tablespaces are on mount points.  I have already received a
> report of such a failure.

I thought it was impossible to use bare mountpoints as tablespaces due
to ownership problems ... Is that not the case?  -1 for special hacks
that work around bogus setups, if that means intrusive changes to the
core code.

> 3)  There is no easy way to analyze all databases.  vacuumdb --analyze
> does analyze _and_ vacuum, which for an 8.4 to 8.5 migration does an
> unnecessary vacuum.  Right now I recommend ANALYZE in every database,
> but it would be nice if there were a single command which did this.

+1 for vacuumdb --analyze-only

> 4)  I have implemented the ability to run pg_migrator --check on a live
> old server.  However, pg_migrator uses information from controldata to
> check things, and it also needs xid information that is only available
> via pg_resetxlog -n(no update) to perform the migration.  Unfortunately,
> pg_resetxlog -n cannot be run on a live server, so pg_migrator runs
> pg_controldata for --check and pg_resetxlog -n for real upgrades.  It
> would simplify pg_migrator if I would run pg_resetxlog -n on a live
> server, but I can understand if people don't want to do that because the
> xid information reported on a live server is inaccurate.

What xid info does it need?  Would it be good enough to use the "next
XID" from most recent checkpoint from pg_controldata?  It is a bit
outdated, but can't you simply add some value to it to have a safety margin?

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.